[wip] etl: zero-copy on mmap

[AskAlexSharov]

 name                    old ns/op    new ns/op    delta                                                                        
  MmapZeroCopy               211643       120180     -43.20%
  BufioRead                  211643       120180     -43.20%                                                                     
                                                                                                                               
  name                    old B/op     new B/op     delta
  MmapZeroCopy               524386         370     -99.93%
  BufioRead                  524386         370     -99.93%